This invention relates to process and apparatus for continuously anodizing aluminum. The term aluminum is used herein to include aluminum base alloys which, like pure aluminun, can be electrolytically anodized to form oxide coatings. More particularly, this invention relates to a technique for continuously anodizing coils or lengths of aluminum, such as aluminum sheets, strips, wire, rods, shapes and the like (hereinafter collectively referred to as aluminum web), by introducing direct current in a cathodic contact cell into an aluminum web passing therethrough having an oxide coating thereon formed in an anodizing cell which precedes the contact cell. The direct current is picked up in the cathodic cell by the already anodized web and is transferred therealong countercurrent to the direction of movement of the web to the anodizing cell in which the oxide coating is formed.
Aluminum and aluminum base alloys in sheet, strip and wire form have been continuously anodized by a number of techniques for many years. Such anodized products are used for electrical and decorative purposes, in the manufacture of household appliances, automotive trim, building materials, farm equipment, furniture, sporting goods, cans, container closures, lithographic plates, transformers, and in many other market and product areas.
Two basic techniques are used to introduce current into a moving aluminum web. The first involves the use of a contact roll or bar and the secnd is an electrochemical technique utilizing a cathodic contact cell.
The contact roll or bar technique suffers from many deficiencies. For example, the aluminum web must be dry to avoid electrolysis which, if it occurs, dissolves the contact roller or bar anodically leaving pits in the surface thereof. .[.Anoter.]. .Iadd.Another .Iaddend.problem is .[.arching.]. .Iadd.arcing .Iaddend.between the two surfaces as they become separated which is brought about by the presence of edge burrs or slivers of aluminum on the web surface itself. Arcing causes pitting of the aluminum as well as pitting and oxidation of the contact member itself.
When using the cathodic contact cell technique, one of the limits on how much current can be introduced into the web is the fact that all of the current has to be introduced into a cross-sectional area of the moving web. This causes a surge of current into the unanodized web which is unprotected by an oxide coating. This tends to cause burning and results in the formation of unsound oxide coatings. Up to now the problem of arcing in the use of a solid contact member and the problem of burning due to a surge of current in the contact cell technique have been accepted as inherent limitations in the continuous anodizing of aluminum for the reason that all of the current for the anodizing operation has to be picked up by the moving web in one pass. The reason advanced for this is the fact that the anodic oxide coating formed in the anodizing operation is an electrical insulator.
The present invention now makes it possible to introduce anodizing current into more than one cross-section of the moving aluminum web and imparts to the moving web the means to avoid burning caused by a surge of current into the moving web as it enters the anodizing cell.